Digital printing technologies have been increasing the utility value in on-demand printing markets and printing markets requiring a small number of documents in recent years. In particular, full-color printing using an electrophotographic technology holds superiority in the productivity, printing cost, ease of maintenance, etc. over other printing technologies and has been rapidly expanding its market.
Both full-color printing using toner of four colors (cyan, magenta, yellow, and black (CMYK)) by the electrophotographic technology, and multicolor printing using special toner, are receiving attention and come within the range of special printing markets requiring a higher level of on-demand features and immediacy.
Multicolor printing using special toner is exemplified by a system of forming an image with transparent toner on a sheet of paper that is subjected to electrophotographic printing, and using the information in the image to prove that the document is an original. Although the printed information is not normally invisible, the printed information becomes visible by irradiating it with ultraviolet light, and it is possible to indicate that the print is not falsified or is not reproduced by copying.
As described above, using special toner can provide new added value different from the value of the normal digital printing to further expand the world of digital printing.
Although the transparent toner layer is superimposed on a black-and-white print in the above example, applying the transparent toner in color printing in the same manner as in the black-and-white printing achieves a similar effect. For example, Japanese Patent Laid-Open No. H10-055085 discloses a technology of using the transparent toner in the color printing to control the glossiness of a printed surface.
This type of multi-color printing using the transparent toner etc. differs from the four-color printing in that the total amount of toner used in the printing is greatly increased.
Particularly in the application of special toner to color printing using the electrophotographic technology, in addition to a toner image with the toner of the four colors C, M, Y, and K, a special toner image with the transparent toner or the like is formed on an intermediate transfer unit, and it is necessary to transfer, heat, and fix the special toner image on a sheet of paper. Since an increased total amount of toner is processed in each electrophotographic process, a heavier load is applied to each electrophotographic process.
In order to lighten the load on each electrophotographic process, after a series of processes including the development, transfer, and fixing of the toner of the four colors C, M, Y, and K has been performed, the same series of processes using the special toner, such as the transparent toner, may be performed on the same sheet of paper. However, with this structure, it is necessary to perform the same series of processes multiple times, and thus the productivity of the apparatus is reduced.
In an electrophotographic process using the four colors C, M, Y, and K, the color components of three CMY colors are substituted with the K (black) color component for every pixel in order to restrict the amount of toner. However, since the special toner cannot be replaced with another toner component, it is not possible to perform the same process when the special toner, such as the transparent toner, is added.
In addition, although the amount of toner is conventionally controlled for every pixel, applying the same process to the printing of additional image information with the toner having a special function, such as the transparent toner, not only increases the amount of calculation but also makes the relative amount of the transparent toner for every pixel unstable to possibly decrease the stability of the additional information.
In the technology disclosed in Japanese Patent Laid-Open No. H10-055085, in order to keep the gloss of a print constant, different amounts of the transparent toner are used for different kinds of paper to control the glossiness for every pixel. However, the control of the amount of toner per unit area is not performed in this technology.